Adsorption Properties of Iron and Iron−Manganese Catalysts Investigated by in-situ Diffuse Reflectance FTIR Spectroscopy

The adsorption properties of the precipitated Fe and FeMn catalysts were studied by in-situ diffuse reflectance FTIR using CO, NO and CO + H2 (syngas) as probe molecules. CO and NO adsorption on the oxidized Fe sample reveals the presence of Fe(III) and Fe(II) on the surfaces. On the H2-reduced Fe sample, the adsorbed probes mainly dissociate because of the high reactivity of the formed fine Fe(0) particles. Upon reduction in syngas, significant amount of carbonaceous and hydrocarbon fragment species is deposited on the surfaces which prevents from the access of the probe molecules to the metallic iron species. But, the spectra recorded from the thermal desorption of the adsorbed syngas displays three desorption bands with their frequencies nearly coinciding with those of the C−O vibrations in Fe3(CO)12 iron carbonyl, claiming that the size of the formed Fe(0) particles is indeed very small. With addition of manganese to the sample, the Fe(II) species dominates on the surface. After reduction either in H2 or in syngas, the Fe(II) and Fe(0) sites are indicated by NO adsorption. CO and syngas adsorption gives rise to a large number of distinct bands corresponding to linear and bridged adsorption species. The bands of CO adsorbed on the deep hollow sites of Fe(0) implies that the size of the Fe(0) particles formed on the reduced FeMn sample is larger than that on the reduced Fe one. The adsorption features of the Fe(0) particles formed on the FeMn samples reduced either in syngas or in H2 (without formation of iron carbides) are quite similar, suggesting that the adsorption properties of the reduced FeMn samples are not determined by the formed iron carbides, but rather by surface Fe(0) particles of different sizes.